US 3215747 A
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Nov. 2, 1965 A. H. FAINBERG ETAL 3,215747 METHOD OF SEPARATING TRIFLUOROETHYLENE Filed May 31, 1965 FROM TETRAFLUOROETHYLENE 4 Sheets-Sheet l Nov. 2, 1965 A. FAINBERG ETAL 3215747 METHOD OF SEPARATING TRIFLOROETHYLENE FRM TETRAFLUORETHYLENE L BY MURRAY HAUPTSCHEIN ATTORNEY United States Patent O This invention relates to the separation of relatively small amounts of trifluoroethylene trom tetrafluoroethylene by selective adsorption techniques.
In the production of tetrafluoroethylene by the high temperature pyrolysis of fluoroform (CHF or difluorochlorornethane (CF HCI) the raw product gases often contain small amounts of trifluoroethylene,
(CF =CHF) The amount of trifluoroethylene produced may range from trace amounts up to about 2% by Weight (based on the Weight of the CF =CF +CF =CFH mixture).
In the polymerization of tetrafluoroethylene to high molecular polymers suitable for molding into end products of high thermal stability and chemical inertness it has been found that it is of great importance to reduce the concentration of trifluoroethylene in the monomeric tetrafluoroethylene to very low values such for example as to values of less than 40 parts per million and preferably less than parts per million and even as low as one part per million or less by weight of trifluoroethylene. Concen trations of trifluoroethylene greater than about 10-40 p.pm. deleteriously affect the properties of the polymer, in particular its thermal stab-ility. Polytetrafluoroethylene prepared from monomer containing excessive amounts of trifluoroethylene tends to ethermally degrade at the relatively high temperatures necessary for fabrication of the polymer into molded shapes resulting, e.g. in undue porosity, loss of tensile strength, loss of dielectric properties, etc.
Because of the closeness of their bolling points and their close similarity in other respects, the separation of trifluoroethylene from tetrafluoroethylene down to such low residual trifluoroethylene concentrations is very diflicult by known methods. Thus, methods such as fractional distillation, -and liquid-liquid extraction and the like are irnpracticable for this purpose. Because of the high reactivity of tetrafluoroethylene, methods involving selective reaction of the trifluoroethylene to a more easily separable compound are likewise difficult or impossible t0 apply.
While it has been found that selective adsorption techniques using many common adsorbents such as silica gel, activated carbon and activated alumina, are capable of making the required degree of separation, we have discovered as will be shown below in more detail, that such adsorbents have such a low capacity in the adsorption process that their use is not economically attractive. Thus, adsorbents of these types, even at relatively low concentrations of trifluoroethylene such as one-half percent, are able to process only 0.1 te 0.5 pound of tetrafluoroethylene per pound of adsorbent before exhausting their adsorpti-on capacity.
In recent years, a relatively new class of adsorbents have corne into use, partcularly in the field of selective 3,215,747 Patented Nov. 2, 1965 hydrocarbon separation, consisting of crystalline metal aluminosilicates (often called zeolites) which in the dehydrated form have a three dimensional network of aluminum and silicon oxides forming intracrystalline voids interconnected by pores of uniform size, often referred to as zeolitic molecular sieves. The use of such adsorbents has been previously suggested for the separation of vinyl fluoride trom vinylidene fluoride (see U.S. Patent 2,917,- 556 to Percival). In this previous work however it was found that the adsorption capacity of these materials in this separation was quite low. As will be shown in more detail hereafter, even at relatively low concentrations of vinyl fluoride, the capacity of the adsorbent was only about /2 pound of vinyl fluoride-free vinylidene fluoride per pound of adsorbent. This is about the same order of capacity that we have found for silica gel, activated carbon and activated alumina in the separation of trifluoroethylene. It was also found by Percival (U.S. Patent 2917,556) that the capacity of the adsorbent was essentially independent of the initial concentration of vinyl fluoride, i.e virtually the same adsorption capacity, in terms of weight of pure vinylidene fluoride produced per weight of adsorbent, was obtained at both low and relatively high concentrations of vinyl fluoride.
It has now been found in accordance with the present invention that, in surprising contrast to the behavior of the vinyl fluoride-vinylidene fluoride system as reported by Percival, the molecular sieve type crystalline metal aluminosilicates have a very high capacity for the separation of trifluoroethylene from tetrafluoroethylene (in many cases of the order of ten to thirty times greater than that for the separation of vinyl fluoride from vinylidene fluoride).
Furthermore, it has been found that such adsorbents are particularly eflective for the selective adsorption of trifluoroethylene from mixtures of CF =CF and at relatively low initial concentrations of CF,=CHF viz. at initial CF =CHF concentrations about 2% and less. Thus, it has been found that in contrast to the work reported by Percival for the vinyl fluoride-vinylidene fluoride systern the capacity of molecular sieve type aluminosilicate adsorbents in the trifluoroethylenetetrafluoroethylene system increases sharply at relatively low concentrations of trifluoroethylene (below about 2%) thus making the use of such adsorbents particularly attractive in the substantially quantitative removal of 2% and less of trifluoroethylene frorn tetrafluoroethylene.
The adsorbents used in the present invention may be described generally as crystalline metal aluminosilicates which in the dehydrated form have a stable three-dimensional netwerk of SiO -and AIO, tetrahedra providing intracrylstalline voids which are interconnected by access openings or pores of uniform size. The eflective pore diameter should be at least about 5 A. (angstrom units). The electrovalence of the tetrahedra containing aluminum is balanced by the inclusion in the crystal of a cation, in particular, alkali metal or alkaline earth metal cations, especially sodium, potassium and calcium ions. The total void volume after dehydration is generally of the order of about 50%. These adsorbents are often referred to generally as zeolitic molecular sieves.
While there are a number of natural crystalline zeolites such as chabazite which have the above type of crystal structure and which rnay act as molecular sieves, most of these natural materials are unavailable in commercial quantities in sutficiently pure form, and in addition most have etective pore diameters which are too small for use in the invention. For this reason the synthetic zeolitic molecular sieves -are much preferred for use in the present invention. These synthetic materials and their method of manufacture are described in detail in both publications and in the patent literature. See for example I Iersh Molecular Sieves, Reinhold Publishing Corporation (1961) chapters 57; Breek et al., J.A.C.S. vol. 78 pp. 5963-5977; and U.S. Patents 2882,243 and 2,882,244.
The type of synthetic zeolitic molecular sieves descrbed in U.S. Patents 2882,243 and 2882,244 are particularly suitable for use in the invention. Adsorbents of these types are commercially available e.g. from the Linde Division of Union Carbide Corporation under the designations eg. Molecular Sieve Types 5 A, X and 13 X.
The preferred sieves are those in which the interstitial metal cations are alkali metal cations or those in which the original alkali metal cations have been replaced in whole or in part by alkaline earth metal cations. Particularly suitable are those in which the metal cations are sodium or calcium ions or both.
As pointed out previously the adsorbents used in the invention should have pore openings (i.e. the openings givng access to the intracrystalline voids) with an effective pore diameter of at least about 5 A. The effective pore diameter refers to the crtical size of the smallest molecule which will be admitted through the pores as distinguished from the theoretical pore diameter formed by the framework of silica and alumina tetrahedra. Thus, Linde Molecular Sieve 5 A (for detailed description see Exarnple II) has an effective pore diameter of about 5 A, will admit both tetrafluoroethylene and tri- ?luoroethylene, and is highly efective in their separation. Linde Molecular Sieve 4 A on the other hand has an effective pore diameter of about 4 A and will admit neither of those olefins and is ineffective in their separation. There is on the other hand no critical upper limit in the efiective pore size of the adsorbent. Thus, Linde Molecular Sieves 10 X and 13 X (tor detailed descrip tion see Examples IV and I respectively) having effective pore sizes of about 10 A and 13 A respectively, are both highly effective in separating trifluoroethylene from tetrafluoroethylene in accordance with the inventon. As is apparent from the foregoing, the separation process of the invention does not depend upon the screenng of the two olefins according to size. Both enter the pores and are adsorbed on the surfaces of the intracrystalline voids as indicated by the fact that both trifluoroethylene and tetrafiuoroethylene are taken up by sieves having effective pore diameters large enough to admit them in amounts of the order of 10 to 40% by weight of the adsorbent. Such a large take up could only be accounted Eor by the adsorption of the olefins on the large internal surfaces provided by the intracrystalline voids. The area of the internal surfaces are generally of the order of 600-800 square meters per gram in contrast to an external area of only 1 to 3 square meters per gram. Rather than depending upon screenng by size, the separation of the two olefins according to the invention depends upon a differental adsorption effect with the tnifluoroethylene neing more strongly adsorbed than the tetrafluoroethylene. In this connection it is most surprising that two naterials as closely related as tetrafluoroethylene and :rifluoroethylene should exhibit such a large difierential 1dsorption (as demonstrated by the high capacity of the adsorbent for removing trifluoroethylene) particularly 2vhen the vinyl fluoride-vinylidene fluoride system exhibts such a low diferential adsorption effect as demon ;trated by the low capacity of the same adsorbents for ;eparating vinyl fluoride from vinylidene fluoride.
The zeolitic molecular sieves are, of course, used in the activated anhydrous form, de. the crystal water has been driven off leaving intracrystalline voids. Suitable sieves are available in activated, essentially anhydrous (no adsorbed water) condition, containing only about 1 weight percent adsorbed air. If water is adsorbed prior to use by exposure, for example, to a humid atmosphere, the adsorbed water can be readily removed by heating the sieves at a temperature of the order of 350 C. While evacuating to low pressure-or sweeping with a purge gas, e.g. air or nitrogen.
The zeolitic molecular sieves will be employed in any convenent physical form such as a powder or in the form of pellets. The pellet form is preferred from the stand point of avoiding undue pressure drops through the system, for uniformity of flow, and case of handling. Generally, pellets ranging from 5 to 4" in size will be found satisfactory. The pellets are usually prepared by formulating the zeolite with about 20% by weight of an inert binder and then compressing the mixture into pel- Iets.
The capacity of an adsorbent in the separaton of a two component mixture A+B where B is the more strongly adsorbed component and is present in relatively minor amount, can be conveniently expressed in terms of the weight of pure A (i.e. A free from B) that can be obtained per unit weight of evacuated adsorbent at the time of initial breakthrough. By evacuated adsorbent is meant adsorbent that contains no adsorbed component. The capaoity value in these terms is designated by the symbol X. The time of initial breakthrough (t is the time which elapses after starting the flow of the A+B mixture through a column of the adsorbent until the first trace of component B is detected in the column effluent. The value of t is readily determined experimentally by monitoring the composition of the effluent to determine the time of appearance of the first trace of B. The valve of X is calculated from the following relationship.
l i i w where u. is the rate of feed of A+B to the exacuated adsorbent in grams per minute; 1 is the time of: initial breakthrough in minutes; w is the total weight in grams of A+B which is adsorbed on w grams of adsorbent at t and where w is the weight in grams of the evacuated adsorbent. The value of w can be determined experimentally by measuring the weight gain of the evacuated adsorbent at 21.
The capacity of the adsorbent in the separation of a two component mixture A+B where B is the more strongly adsorbed component and is present in relatively minor amount, canalso be expressed in terms of the weight of effluent obtained per unit weight of adsorbent at the time of 50% breakthrough. The capacity value in these terms is designated by the symbol Y. The time of 50% breakthrough is the time which elapses after starting the flow of the A+B mixture through a column of the adsorbent until the concentration of B in the effluent is 50% of its concentration in the feed mixture of A+B. The value of t is readily determined experimentally by monitoring the compostion of the effluent to determine when the concentration of B has risen to 50% of its concentration in the feed composition. The value of Y is calculated from the following relatonship:
where U, t and w are as defined above and where w is the total weight in grams of A+B which is adsorbed on w grams of adsorbent at I The value of w can be determined experimentally by measuring the weight gain of the evacuated adsorbent at t The advantage of expressing the capacity in terms of Y (capacity based on 50% breakthrough, (t rather than in terms of X (capacity at initial breakthrough (t is that the value of Y more closely represents the maximum capacity value that can be attained in actual practice. Furthermore, the value of Y is virtually independent of feed velocity, column length, ratio of the column length to diameter, adsorbent particle size, adsorbent particle sze distribution and adsorbent packing density.
As pointed out above, the value of Y closely approximates the maximum purification capacity attainable (in terms of weight of pure component A obtainable per unit weight of adsorbent). This results from the fact that the capacity at 1 for most systems is essentially equivalent to the equilibrium purification capacity Y which is defined as the weight of pure component A obtainable per unit weight of adsorbent when the adsorbent is operated to the point at which it is unable to adsorb further quantities of component B, that is until the composition of the efliuent from the adsorbent is the same as the composition of the feed. At this point the adsorbent is in equilibrium with the feed. The relationship of Y and Y can be better understood by reference to FIGURE 1, where the concentraton of component B in the effluent from the adsorbent, expressed as the percent of its concentration in the feed, is plotted against time. From time t (ie. the start of feed to the activated adsorbent) to t (time of initial breakthrough) the concentration of component B in the efliuent is zero. During this time pure A is recovered. At t the first trace of B appears in the eflfluent and from t to t the concentration of B increases at the rate shown by curve 1 until at t the concentration of B in the efliuent is 100% of its concentration in the feed. At this point the adsorbent is in equilibrium with the feed gas and no further separation of B from A will occur. At t the concentration B in the eflluent is 50% of its concentration in the feed. The amount of component B which has appeared in the eflluent at t is proportional to the shaded area D while the residual capacity of the adsorbent to remove component B from the mixture A+B at t is proportional to the shaded area E. If curve 1, defining the rate at which the concentration of B increases with respect to time, is symmetrical, as it is in FIGURE 1, then elapsed time t to t is the same as t to t and the area D is equal to area E. In these circumstances, the amount of. B which has appeared in the eflluent at (represented by area D) is the same as the amount of B that can still be adsorbed by the adsorbent at t (represented by area E). Thus, when curve 1 is symmetrical the maximum capacity, viz. the equilibrium capacity Y of the adsorbent (in terms of weight of pure A obtainable per weight of adsorbent) becomes equal to Y. Since in most cases curve 1 is approximately symmetrical in shape, Y can be generally taken for practical purposes as representing the maximum capacity (i.e. equilibrium capacity) of the adsorbent. Even in cases where curve 1 is somewhat a symmetrical the value of Y will generally closely approximate Y EXAMPLES I T0 IV The following Examples I t0 IV illustrate the separation of trifluoroethylene from tetrafluoroethylene by selective adsorption on various types of zeolitic molecular sieves according to the invention and demonstrate the high purification capacities attainable. In each exarnple the adsorbent in the form of pellets containing about 20% by weight of an inert bonding material is loaded into a vertical tube with a 40 cm. length of packed section and an inside diameter of 2 om. (lengthzdiameter ratio of 20:1). The evacuated (no adsorbed component) weight of the adsorbent in each case is shown in Table I. In Example I, the adsorbent is a zeolitic molecular sieve of the type described in U.S. Patent 2,882,244 supplied by the Linde Division of Union Carbide Corporation under the designation Molecular Sieve Type 13 X. It has the general chemical formula 6 It has the X crystal structure which is cubic, a =2495 angstroms, space group and is characterized by a 3-dimensional netwerk of A10 and Si0 tetrahedra which after removal of crystal water form mutually connected intra-crystalline voids accessible through openings (pores) which will admit molecules with critical dimensions up to 9 A. The void volume is 51 vol. percent.
T able l Example I II III IV Type of adsorbent 13X 5A AW-500 10X Evacuated weignt of adsorbent, gms.-. 69.3 76. 3 73. 1 69. 4 Weight Percent CF2=CHF in feed 0. 45 0. 41 0. 41 0. 43 Mess fiow of feed, grams/mnute 2. 2. 94 3. 00 2. 92 Superfieial linear velocity, ft.lsec.- 0.129 0.128 0.131 0.128 Initial breakthrough time, t, min. 286 6 99 50% breakthrough time, t5, min 357 247 153 148 Weight of OF3=CF2+OF2=CFH on adsorbent after t, gms 18. 2 16. 8 10. 4 17. 2 Purifieation. capaeity X (i.e. at ti) 11. 9 5. 8 2. 5 3. 9 Purification capacity Y (i.e. at t5) 14. 9 9. 3 6. 3 6. 0
with an average volume of voids of about 0.38 cubic centimeters per gram. It has an internal surface area of 650-800 square meters per gram and an external surface area of 1 to 3 square meters per gram.
In Example II, the adsorbent is a zeolitic sieve of the type described in United States Patent 2882,243 supplied by the Linde Division of Union Carbide Company under the designation Molecular Sieve Type 5 A. This adsorb ent is prepared from Linde molecular sieve type 4 A by exchanging (through ion exchange) about 75% of the sodium ions of Linde Molecular Sieve Type 4 A for calcium ions. Linde Molecular Sieve Type 4 A, which is also of the type described in United States Patent 2,882243, has the general chemical formula Molecular sieve type 5 A has the A crystal structure which is cubic, a =12.32 A., space group characterized by a three dimensional network of A104 and 810 tetrahedra which after removal of crystal water form mutually connected intra-crystalline voids accessible through openings (pores) which will admit molecules with critical dimensions up to about 5 A. in diameter. The void volume is about 45% with an average volume of voids of 0.27 cubic centimeters per gram. It has an internal surface area of 650 to 800 square meters per gram and external surface area of 1 to 3 square meters per gram.
In Example 111, the adsorbent is a zeolitic molecular sieve supplied by the Linde Division of Union Carbide Company under the designation Molecular Sieve Type AW-500. This sieve is similar to the adsorbents used in Examples I, 11 and IV, being characterized by a three dimensional network of A10 and Si0.; tetrahedra which after removal of crystal water form mutually connected intra-crystalline voids accessible through openings (pores) which will admit molecules with critical dimensions of slightly less than about 5 A. It is resistant to acid attack and thus is specially adapted for use in acidic environments.
In Example IV, the adsorbent is a zeolitic molecular sieve of the type described in United States Patent 2,882,- 244 supplied by the Linde Division of Union Carbide Company under the designation Molecular Sieve Type 10 X. This is prepared from Linde Molecular Sieve Type 13 X, described above, by exchanging (through ion exchange) about 75% of the sodium ions of molecular sieve type 13 X for calcium ions. Type 10 X has the name crystal structure as Type 13 X and approximately he same vid volume, but the pores are somewhat small- :r and will admit molecules only with critical dimensions lp to about 8 A. It has an internal and external surface .ICEL similar to that for Type 13 X.
In each of the examples, a mixture of tetrafluoroethylme and trifluoroethylene containing a concentration of rifluoroethylene as shown in Table I is introduced into he bottom of the vertical column of adsorbent at ambient emperature (25 C.). The pressure in the system is 1pproximately atmospheric (15 p.s.ia). The adsorbents tre employed as received in their air-loaded condition, the 1dsorbed air being immediately displaced when the flow )f the mixture is started. The mixture is introduced into he column at a constant mass flow rate and superficial inear velocty as shown in Table I for each of the four uns.
The composition of the eflluent gas from the column is :ontinuously monitored by passage through a gas Sam )ling valve of a gas chromatograph. By sampling and 1nalyzing the eflluent at frequent intervals, the time of nitial breakthrough (t and the time of 50% breakhrough (t is determined for each run. The product Trom the adsorbent is collected in a liquid nitrogen cooled 'eceiver.
At the outset, while the air is being desorbed and re- )laced by tetrafluoroethylene, the temperature of the :olumn in the zone where desorption and replacement is aking place increases, and the warm zone travels up the :olumn. After displacement of the air, the column cools 0 ambient temperature (25 C.) and continues at this emperature during the remainder of the operation. The teat of displacement of the tetrafluoroethylene by the rifluoroethylene is too small to exert any significant emperature elect because of the relatively small percenttges of trifluoroethylene in the feed.
In each case, the time (t,) of initial breakthrough of F CHF is measured. This is taken at the time when he concentration of CF =CHF in the effluent exceeds hirty parts per million. The time of 50% breakthrough t is also measured. The values of w (total amount )f CF =CF and CFF-CFH adsorbed at t and the alue of w (weight of CF =CF and CF =CFH on he adsorbent at t are measured by the weight gain of he adsorbent. In practice, it is not necessary to measu.re
is of the order of 3 to 12 grams of trifluoroethylene-tree tetrafluoroethylene per gram of adsorbent.
The very high purification capacities attainable in accordance with the invention becorne apparent when compared to the purification capacities obtainable with other commonly used adsorbents such as silica gels, activated carbons and activated alumina. In the following examples (Examples A, B, C, D and E), a series of tests were made under conditions similar to these used in Examples I to IV to determine the capacity of silica gel, activated carbon, and activated alumina for the removal of trifluoroethylene from tetrafluoroethylene by selective adsorption. In each example, the adsorbent in the form of small pellets or granules is loaded into a vertical tube with a 40 cm. length of packed section and an inside diameter of 2 cm. (lengthzdiarneter ratio of 20:1). The evacuted (no adsorbed component) weight of the adsorbent is shown in Table II. In Example A the adsorbent is a granular silica gel having a total surface area of the order of 800 square meters per gram. In Example B, the adsorbent is a silica gel in the form of beads having a surface area of the order of 600 square meters per gram. In Example C, the adsorbent is an activated carbon obtained by the destructive distillaton of bituminous coal and having a total surface area of the order of 1000 to 1200 square meters per gram and a pore volume of about 0.8 cubic centimeters per gram. In Example D, the adsorbent is an activated carbon obtained by the destructive distillation of coconut shells having a surface area similar to that of Example C. In Example E, the adsorbent is activated alumina consisting over 99% of alumina and low in sodium, iron and silica, and having a surface area of about 230 square meters per gram. A feed mixture of tetrafluoroethylene containing a small amount of trifluoroethylene is introduced into the bottom of the column at the mass flow rate and superficial linear velocty shown in Table II. The time at initial breakthrough and at breakthrough are measured in the manner previously described. The weight of tetrafluoroethylene plus trifluoroethylene adsorbed after t, is also determined in the manner described above. The purification capacity X (at t and Y (at 1 is shown in Table II. As is apparent, the purification capacities obtainable with the use of the zeolitic molecular sieve type adsorbents is 20 to 100 times greater than with these other types of adsorbents.
T able II Example A B 0 D E Type of adsorbent Silica gel Siliea gel Activated Activated Activated Mass flow of feed, grams/rninute Superficial linear velocty, ft./see
Initial broakthrough time t minnfe 50% breakthrough time tro, minnh= Weight of CF;= CF +CF CI-IF on adsorbent after t, gms
Purillcation capacity Y (i.e. at 1150) Purification capacity X (i.e. at t) 0.1
hese weight gains precisely at t and t After t there s only a very small change in the total weight of As mentioned previously, an important feature of the invention is the high capacity of the zeolitic molecular sieves for the removal of low initial concentrations of trifluoroethylene, viz. about 2% by weight and less, trom tetrafluoroethylene. The manner in which the capacity of several types of zeolitic molecular sieves varies with respect to the initial concentration of trifluoroethylene in mixtures of CF =CF and CF =CFH containing from 0 to 6% of trifluoroethylene is shown in FIGURES 2 and 3. In FIGURE 2, curves 2, 3 and 4 show respectively the manner in which capacity varies with initial concentration of trifluoroethylene for molecular sieves Type 10 X, Type 5 A and Type 13 X, where capacity is expressed in terms of Y (i.e. at t In FIGURE 3,
=xpressed in terms of X (capacity at initial breakthrough) curves 5, 6 and 7 show respectively the manner in which 9 capacity varies with respect to inital trifluoroethylene concentration for molecular sieves Type 10 X, Type 5 A and Type 13 X, where capacity is expressed in terms of X (i.e. at t As is apparent from these curves, at initial trifluoroethylene concentrations over about 2% by weight, the capacity varies only siightly with respect to trifluoroethylene concentration, while in the case of trifluoroethyleue concentrations of less than 2% capacity rises rapidly. The enhanced capacity of the zeolitic sieves for the rernoval of trifluoroethylene frorn tetrafluoroethylene at 10W initial concentrations of trifluoroethylene of 2% and 1ess makes the process of the invention of particular value in this concentration region. Thus, the process of the invention is of great value in the ultra-purfication of tetrafluoroethylene by the removal of 2% or less, and particularly for the removal of 1% or less, of trifluoroethylene trom mixtures of CF =CF and CF =CFH down to 10W values of the order of 20 parts per million (by weight) and preferably to less than 10 parts per million of residuai trifluoroethylene.
The enhanced capacity of the zeolitic sieves for the removal of trifluoroethylene frorn tetrafluoroethylene at 10W trfluoroethylene ooncerrtrations is in surprisng contrast to the behavior displayed by vinylidene fluoridevinyl fluoride mixtures using the same type of adsorbents, as reported by Percival in United States Patent 2,917,556. Reference is made to 'Irab1e II which shows the capacities obtained by the use of zeolitic molecular sieves for the renroval of vinyl fluoride fnom vinylidene fluoride in Examples 1 to 3 reported in United States Patent 2,917,- 556. The purification capacities obtained in the remaining examples of the patent are indicated to be substantially the same as these reported for Exampies 1 to 3. The purification capacities are expressed in term of X (i.e. at t snce this value can be calculated from the data reported by Percival, and is directly comparable to the value shown herein fior the CF =CF +CF =CFH system. It is apparent first of all that the purification capacites determinedfrom Percivals data are much 1ess at all impurity concentrations than those obtained for the CF =CF +CF =CFH system in accondance with the invention. In the area of particular interest (i.e., impurity concentratons of about 2% and less) the differenccs between the respective purification capacities of the two systems are greatly different. For example, at an impurity level of about 0.5% the capacities obtained for the CF =CF +CF =CFH system in accordance with the invention are more than twenty t1imes greater than for the CF =CH +CFH=CH system. In the second place, it can be seen that the capacity of the zeolitic sieves is essentially constant for the CF =CH -ICFH=CH system irrespective of the initial concentration of vinyl fluo- T able III REMOVAL OF VINYL FLUO RIDE FROM VINYLIDENE FLUO RIDE USING ZEOLITE MOLECULAR SIEVES (DATA FROM U.S. PATENT 2917,566W. O. PERCIVAL) Percivais Exampie N 1 2 3 Type of Adsorbent 13X 13X 13X Evacuated weght of adsorbent, grns. 742. 5 708 742. 5 Concentration of CH CHF in ieed, mol percent 6. 4 0. 61 23. 2 Concentration of CH2=CHF in feed, wt. percent. 4. 7 0.44
Superficial iinear velocity, feet per sec. Mass flow of feed, grams per minute. Initiai breakthrough time, t, minutes. Weight of OH =CF +CH =CHF adsorbed after ride in the mixture. Enhanced oapacity a-t relatively 1ow concentrations of 2% and less is not obtained as in the case of the present invention. This is illustrated clearly in FIGURE 3 where curve 100 shows the manner in 10 which the pun'fioation capacity X in Percivals systeni CF =CH +CFH=CH vares with concentration at vinyl fluoride concentratioug between about 0.5 110 6%. As may be =seen the capacity is virtually unchanged over this range.
In general, the adsorption separation process of the invention may be carred out at temperatures ranging from about 50 C 10 +50 C. and preferably from 30 to +30 C. It has been fourrd that the cap;acity of the zeolitic molecular sieves increases at lower temperatures and a partcularly preferred range of operatng temperature is from 20" to [20 C.
Examples V to VII nclusive which are summarized in Table IV, show the variation of purification capacity with respect to temperature for a Type 10 X molecular sieve at temperatures from +20 to 26 C. These examples Were carrierd out in the same manner and using the same equipment as in previous Examples I to IV inclusive. As -may be seen, the purification :capacity Y (i.e., at t of the sieve increases from 6.0 gr-ams of trifluoroethylenefree tetrafluoroethylene per gram of adsorbent at [25 C. to a capacity of 16.0 at 26 C. The variation of capacity with respect 'o temperature for the Type 10 X sieve is shown graphically in FIGURE 4 where curve 8 shows the variation of capacity with temperature at constant pressure at temperatures of 30 10 +30 C.
Aside from the fall ofl in capacity as the temperature increases, it is also desirable to avoid temperatures over about +30 C. from the standpoint of minimizing the tendency of tetrafluoroethylene to polymerize on the zeo litic sieves.
Table 1 V Example V VI VII Type of adsoxbent 10X 10X 10X ernp C 25 0 26 Pressure, p.s.ia 15 15 15 Evacuated weight of adsorbent, grams. 69. 4 69. 1 69.9 Weight percent of CF2=CHF in feed 0.43 0. 42 0.42 Mass flow of feed, grams per minute 2. 92 2. 3. 03 Superfieiai linea.r velocity, ft./sec 0. 128 0. 114 0. Initial breakthrough time, t, minut 99 181 322 50% breakthrough time, t5 minutes 148 253 37 Weight of CF2=CF2+CF2=CFH on adsorbent aiter t gms 17. 2 19.0 21. 0 Purification capacity X (i.e. at t) 3.9 7. 2 13. 7 Purification eapaeity Y (i.e. at t o) 6.0 10. 2 16.0
The pressure employed during separation is not critical in the sense of determiniug whether or not the separaton will take place. Thus, subatmospheric pressures, normal pressures and super-atmospheric pre ssures may be ernployed. Super-atmospheric pressures greater than 300 p.s.ina. (-.pounds per square inch absolute) are preferably avoided because of the greater tendency of tetrafluoroethylene to polymerze on the sieves at such pressures. Although sub-atmospheric pressures as 10W, for example as 5 p.s.i.a. may be employed if desired, it is =generally more convenient to operate at normal or moderate superatmospheric pressures. Aflthough it has been found that the capacity of the sieve is not greatly pressure dependent, there is some capacity iucrease as the pressure increases frorn atmospheric to about 50 p.s.i.a. Sirrce the tetrafluoroethylene will ordinarily be handled under pressure it will genera1ly be convenient t o operate at moderate super-atmospheric pressures.
Examples VIII 00 X inciusive, which are summarized in 'Fable V, show the variation in capacity at constant temperature of about 25 C. as the pressure increases from 15 to 75 p.s.a. for a 10 X type molecular sieve. These examples were carried out using the same equip ment and the same procedures as described for the previous Examples I to IV. As may be seen, the purification capacity Y (i.e., at t increased from 6.0 to 7.15. FIG- URE 5 shows graphically the effect of incre.asing pressure on purificaton capacity in the range of 10 to 75 p.s.ia. for a Type 10 X molecular sieve at constant temperature. Curve 9 showing this relatonship illustrates Table V Example VIII IX X Type of adsorbent 10X 10X 10X Temp., C 25 24 25 Pressure, n s i 2 15 29 75 Evacuated wt. of absorbent, gms 69. 4 69. 1 82.3
Weight percent of CF =CHF in feed Mass flow of feed, grams/minute Superficial linear velocity, ft./see- 0. 067 Initial breakthrough time, in, minutes 99 109 131 60% breakthrough time, t5o, minutes 148 160 206 Weight: of CF =CF +OF =CFH on adsorben alter t grams 17. 2 18. 5 24.8 Purification capacity X (i.e. at t;) 3. 9 4. 4 4. 4 Purification capacity Y (i.e. at tw) 6.0 6. 6 7. 15
sorbent to low pressures and elevated temperatures whereupon both of the adsorbed components i.e. tetrafluoroethylene and trfluoroethylene are desorbed. Elevated temperatures of from 150 to 350 C. and preferably from 180 to 300 C. and reduced pressures of one millimeter of mercury or less will generally be used. Where the concentraton of trfluoroethylene in the feed is relatively low i.e of the order of 2% or less, the adsorbent after saturation will contan a relatively high proportion of tetrafluoroethylene. The adsorbed tetrafluoroethylene may be recovered separately, with a desorption of only a small proportion of the trfluoroethylene adsorbed on the sieve, by a stepwise regeneration procedure in which the saturated sieve is first subjected to relatively low temperatures and reduced pressures to remove most of the less strongly adsorbed tetrafluoroethylene and only small amounts of the more strongly adsorbed trfluoroethylene, after which the temperature is raised and/or the pressure decreased to remove trfluoroethylene. The tetrafluoroethylene-rich eflluent from the first stage containing only small proportions of trfluoroethylene may then be recycled to a fresh sieve, and in ths way the tetrafluoroethylne content recovered. For example, a X type molecular sieve after reachng saturation when used for the removal of approximately 0.4% trfluoroethylene from tetrafluoroethylene will release 98% of the adsorbed tetrafluoroethylene and only about 8% by weight of the adsorbed trfluoroethylene when subjected to evacuation to a pressure of 0.2 mm. Hg at a temperature of 25 C. The remainder of the trfluoroethylene may then be removed in a second stage by heating to a temperature of 180 C. at.0.2 mm. Hg for about 1 hour. The sieve is then cooled and subsequently pre-loaded with pure tetra fluoroethylene to the operating temperature and pressure. The sieve, thus regenerated, is highly effective and may be reused for the treatment of further quantities of impure tetrafluoroethylene (i.e. containing trfluoroethylene) to yield tetrafluoroethylene contaning less than 2 p.p.m. of trfluoroethylene.
Other methods that may be used for regeneration of the zeolitic sieves include, for example, selective displacement of the tetrafluoroethylene by passing carbon dioxide through the sieve at room temperature. The displacement of the tetrafluoroethylene by the C0 is essentially quantitative, and only small quanttes of trfluoroethylene are removed, such that the tetrafluoroethylene thus recovered is suitable for recycle to a fresh sieve for recovery of the tetrafluoroethylene content. Following the CO treatment, the adsorbed CO and trfluoroethylene may then be removed by purging with an inert gas such as ntrogen, preferably at an elevated temperature such as 150 C. to 300 C.
Still another regeneration method that may be employed is direct purging with an inert gas such as nitrogen at elevated temperatures of e.g. 150 C. to 300 C. The tetrafluoroethylene is quantitatively removed and the tri fluoroethylene may be removed down to the required level. This latter method however has the disadvantage that the tetrafluoroethylene is dificult to recover from its admixture with nitrogen.
The Examples XI to X111 nclusive which are summarizedin Table VI illustrate the successive regeneration and reuse of a Type 5 A molecular sieve for the purificaton of tetrafluoroethylene containing about 0.4 percent trfluoroethylene by weight. In Example XI a new sieve was employed while in Exarnple XII the same sieve was employed after regeneration by evacuaton to a prcssure of about 0.25 mm. Hg and a temperature of about 250 C. for about minutes. In Example XIII the same sieve was again employed after a second regeneration un der similar conditions. The equpment and procedures employed in these examples were the same as those used in Examples I to IV. As may be seen, the purification capacity dropped only slightly on the first regeneration and remained essentially constant after the second regeneration.
Table VI Example XI XII XIII Type ofadsorbent 5A 5A 5A Number of regenerations- Temp.0
Pnrification eapacty X (i.e. at t) Puri.fication capacity Y (i.e. at t 1 New sieve. 2 First regeneration. Second regeneration.
In large scale commercial operations t will be generally desirable to employ multiple columns of adsorbent arranged in series and suitably manifolded so that they may be successvcly operated to full capacity and successively regenerated. An example of a suitable arrangement is shown in FIGURE 6 wherein reference numerals 10, 11 and 12 refer to the columns of adsorbent. The raw feed (i.e. trifluoroethylene-c0ntaining-tetrafluoroethyl ene) is fed to the columns through line 13 and manifold 14. Manifold 14 is connected to each of the three columns by branch lnes 15, 16 and 17 controlled by valves 15a, 16a and 17a respectively. The purified efluent from the adsorbent is removed by manifold 18 connected to the top of the columns by branch lines 20, 21 and 22 controlled by valves 20a, 21a and 22a respectively. Purfied tetrafluoroethylene is withdrawn from the system by line 19 for any desired use.
T0 regenerate the adsorbent a.fter saturation, the adsorbent columns 10, 11 and 12 are provided with heating and coolng means (not shown) by which the adsorbent can be heated to the desired regeneration temperature and subsequently cooled to operating temperature. Manifold 23 is provided at the top of the columns and is connected to the columns by branch lines 24, 25 and 26 controlled by valves 24a, 25a and 26a respectively for wthdrawng desorbed material from the adsorbent during the regeneration cycle. Vacuum pump 27 is provided to reduce the columns to the desired regeneration pressure. Desorbed material may be removed from the system by line 28 controlled by valve 28a, while portions of t may be recycled to line 13 by line 29 controlled by valve 29a. Compressor 2% is provided to compress the recycled material to the pressure in line 13. At comple- 13 tion of regeneration, the adsorbent columns are cooled down to operating temperature and loaded with pure tetrafluoroethylene to operating pressure.
The top of each column is connected to manifold 33 at the bottom of the columns by branch lines 30, 31 and 32 which are controlled by valves 30a, 31a and 32a respectively. Manifold 33 is in turn connected by branch lines 34, 35 and 36, controlled by valves 34a, 35a and 36a respectively, to the bottom of the columns. By means of lines 30, 31 and 32, manifold 33, and lines 34, 35 and 36, the columns of adsorbent may be selectively interconnected with one another in series as will be described below.
At the beginning of the operation, When all the adsorbent is fresh, the raw feed is introduced through line 13 into column 1 through line 15 and valve 15a While purified eflluent is taken ofi at the top of column through line 20 and valve 20a and removed from the system through manifold 18 and line 19. During this period of operation all other valves are closed.
At some time before column 10 has reached its initial breakthrough point (i.e. before the concentration of trifluoroethylene in the eflluent has reached some predetermined limit such as 10 parts per million) column 11 is placed in series with column 10 by closing valve 20a and opening valves 30a, 3511 and 21a. Flow through column 10 is continued until the adsorbent therein has become fully saturated (i.e. the concentration of trifluoroethylene in its efiluent has reached the concentration in the raw feed). At this point column 10 is ready for regeneration. Valves a, a, a and a are closed While valve 16a is opened to permit the raw feed to pass directly into the bottom of column 11. Column 10 is then placed in the regeneration cycle by heating the column, opening valves 24a and 28a and placing the column under vacuum through vacuum pump 27. At completion of regeneration, valve 29a is closed, and the column is cooled to operating temperature under vacuum, valve 24a is closed, and the adsorbent is loaded with pure tetrafluoroethylene to operating temperatures and pressure through line 20 and valve 20a. While regeneration of column 10 is proceeding, the purification proceeds in column 11 until it approaches initial breakthrough, at which time it is placed in series with column 12 by closing valve 21a and opening valves 31a, 36a and 22a. When column 11 has become fully saturated, it too is shut down and placed in the regeneration cycle in the manner described for column 10.
When column 12 approaches its point of initial breakthrough, it is then placed in series with column 10 by closing valve 22a and opening valves 32a, 34a and 20a. Column 10 by this time, has been regenerated and is ready for further purification duty. When column 12 is completely saturated, it too is shut down for regeneration in the manner previously described.
By thus successively interconnecting the columns in series the entire purification capacity of each of the columns may be -utilized While at the same time obtaining an eflluent of any desired purity. Any number of columns, of course, may be interconnected with one another in the manner shown to undergo successive purification and regeneration cycles.
During the regeneration, the first portions of the effluent, particularly where the regeneration is conducted in stages with an initial low temperature stage, will be rich in the more easily desorbed tetrafluoroethylene. This tetrafluoroethylene-rich, trifuoroethylene-lean portion may, if desired, be recycled to the system for recovery of the desorbed tetrafluoroethylene by line 29. In the second stage of the regeneration where the efllfiluent is principally trifluoroethylene, the regeneration eflluent may be withdrawn from the system by line 28.
The optimum rate of feed of the tetrafluoroethylenetrifiuoroethylene mixture to the adsorbent is readily de 14 termined empirically. Generally, superficial linear velocities in the range of from 0.01 to 1.0 feet per second will be found satisfactory.
Under some conditions, such as operation at atmospheric pressures and reduced temperatures, e.g. 0 C. it will be possible to conduct the separation in the absence of a polymerization inhibitor for the tetrafluoroethylene. Under other conditions, on the other hand, such as operation at superatmospheric pressures, and/ or at somewhat higher temperatures, it will be desirable to conduct the separation in the presence of an inh.ibitor such as dipentene or Terpene B or other terpenes of the type, for example, described in United States Patent 2,407405. Since many of the molecular sieves tend to adsorb such inhibitors at least to some extent, in order to insure the presence of the inhibitor throughout the column of adsorbent, it is preferable to pretreat the ads-orbent with inhibitor, such as by tumbling pellets of the adsorbent in an atmosphere of inhibitor vapor prior to changing the pellets into the adsorbent column. In this manner, the pellets are uniformly treated with the inhibitor such that inhibitor is present throughout the column. The presence of the inhibitor has no substantial effect upon the capacity of the adsorbent for the removal of trifluoroethylene from tetrafluoroethylene.
The separation of the trifluoroethylene from tetrafluoroethylene will occur in the presence of other components that are often found in the pyrolysis product produced by the pyrolysis of fluoroform or CF HCl such as hexafluoroethane, C F octafluoropropane, C F fluoroform, CHF pentafluoroethane, C HF perfluorobutyne-2, CF3CECCF3, perfluoropropene, CF CCF perfluorocyclobutane, c-C.;F; perfluorobutene-Z While the capacity for the removal of trifluoroethylene may be somewhat decreased by some of these impurities, its removal will nevertheless proceed in their presence.
It is to be understood that the foregoing specific embodiments and illustrative exarnples are given by way of illustration and that the invention is not limited thereto.
1. A method for separating trifluoroethylene trom tetrafluoroethylene which comprises contacting a mixture of tetrafluoroethylene and trifluoroethylene containing not more than 2% by weight of trifluoroethylene with a cnystalline metal aluminosilicate having in the dehydrated form a stable, three-dimensional network of SO. and A10 tetrahedra containing interstitial metal cations, said network providing intracrystalline voids intercom nected by pores having an effective diameter of at least about 5 A. thereby preferentially adsorbing said trifluoroethylene on said aluminosilicate, and recovering tetrafluoroethylene essentially free from trifluoroethylene.
2. A method in accordance with claim 1 in which the separation is carried out at a temperature of from 30 to +30 C.
3. A method in accordance with claim 1 in which the separation is carried out at a pressure of from 15 to 300 p.s.1.a.
4. A method for separating trifluoroethylene from tetrafluoroethylene which comprises contacting a mixture of tetrafluoroethylene and trifluoroethylene containing not more than 2% by weight of trifluoroethylene with a crystalline metal aluminosilicate having in the dehydrated form a stable, three-dimensional network of SO and A10; tetrahedra containing interstital metal cations selected from the class consisting of alkali metal and alkaline earth metal cations, said netwerk providing intracrystalline voids interconnected by pores having an eirective diameter of at least about 5 A. thereby preferentially adsorbing said trifluoroethylene on said alumino- 1 5 ilcate, and recoverng tetrafiuoroethylene essentally free from trifluoroethylene.
5. A method in accordance with claim 4 in Which said ;eparaton is carred out at a temperature of from 30 10 -|30 C.
6. A meth0d in accordance with claim 4 in which said separation is carred out at a pressure of from 15 to 30 p.s.1.a.
References Cited by the Examiner- UNITED STATES PATENTS 2917,556 12/59 Percival 260653.3
LEON ZITVER, Prmwny Examiner.
DANIEL D. HORWITZ, Examiner.
UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No 3,215,747 November 2, 1965 Arnold Harold Painberg et al.
It is hereby certfied that error appears in the above numberedpatent requirng correcton and that the said Letters Patent should read as correoted below.
Column l, line 36, for "ethermally" read thermally column 2, line 18, after "roethylene" insert from tetrafluoroethylene column 4, line 37, for "valve" read value column 9, line 28, for "Table II" read Table III same column 9, in the headng to Table III, line 3 thereof, fox "2,917,566 read 2,917,556 column 10, line 16, for "+20" read +25 column 11, Table V, under the heading "Example", line5 5 and 1() thereof, for
"C1 each occurrence, read CF column 12, Table VI, first CQlumn, line 6 thereof, for "CF read CF column 13, line 16, for "column l" read column l() Signed and sealed this 19th day of July 1966.
ERNEST W SWIDER EWARD J. BRENNER Attestng Officer Commissioner of Patents